In the field of geophysical prospecting, the knowledge of the subsurface structure of the earth is useful for finding and extracting valuable mineral resources such as oil and natural gas. A well-known tool of geophysical prospecting is a “seismic survey”. In a seismic survey, acoustic waves produced by one or more sources are transmitted into the earth as an acoustic signal. When the acoustic signal encounters an interface between two subsurface strata having different acoustic impedances, a portion of the acoustic signal is reflected back toward the earth's surface. Sensors detect these reflected portions of the acoustic signal, and outputs of these sensors are recorded as data. Seismic data processing techniques are then applied to the collected data to estimate the subsurface structure.
Seismic surveys can be performed on land or at sea (or other bodies of water). In a typical marine seismic survey, a parallel arrangement of multiple streamer cables or “streamers” are towed behind a vessel, each streamer including multiple seismic sensors positioned at spaced intervals along its length. One or more seismic sources may also be towed behind the vessel and periodically triggered to provide multiple measurements throughout the region of interest.
Because the acoustic impedance of air is much lower than that of water, marine seismic sources are typically submerged to increase the efficiency with which acoustic energy is coupled to the seafloor. Conventional marine seismic sources radiate acoustic energy in directions, i.e., not only within the beam that is useful for detecting reflections from the subsurface, but also in all other directions including the horizontal direction within the water layer and upward towards the sea-surface. Because of this, a significant part of the total amount of energy is emitted in directions that are not useful tier detecting reflections from layers in the subsurface. In addition, energy that is propagating in the upward direction towards the sea surface from conventional seismic sources is reflected down again. The reflection makes it appear as if each source firing is shortly followed by the firing of a “ghost” source, causing the downward-propagating waves to interfere constructively and destructively with each other, thereby degrading measurements.
While the invention is susceptible to various alternative forms, equivalents, and modifications, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto do not limit the disclosure, but on the contrary, they provide the foundation for alternative forms, equivalents, and modifications falling within the scope of the appended claims.